Trimming when coming off the plane

[BrisFisher12]

I'm still fairly new to boating so I hope this isn't a silly question, I've recently repowered my boat with a 100HP 4 stroke from a 90hp 2 stroke. With the new motor when I slow down from being up on the plane and fully trimmed in, I can see a wave start to form behind the motor, if I were to reduce the power quickly I suspect the wave would break over the motor. If I adjust the trim while slowing down I can reduce the effect of the wave and come to a slower stop without any issue.


I don't recall it being an issue with the old motor, I found with the old motor I didn't need to trim the motor in fully when up on the plane. I suspect the weight of the new 4 stroke is the major difference here and I'm OK with that, the new motor is fantastic in all respects.


So my question is this, is it normal to have to trim the motor out while slowing down? Or am I missing something?

[stockhorse]

Are you traveling at the same speed on plane with the new motor as you did with the old motor? If you are traveling faster when you drop of the plane you will push more water and actually be reducing speed faster and that will cause a following wave.

[Chimo]

Hi

You recognize that the new motor is a fair bit heavier and your still happy so in reality you probably need to slow down more gradually to avoid the following wave and reduce the chance of water washing over the stern. And; see below.........

Why do you think you need to trim the new motor in vs the old one?
Are you running bow up without doing this?
If so you perhaps have so much more weight that you may need to consider some changes to get some more rear lift.

What is the boat?

Do you have trim tabs on the boat?

Do you have foils on the motor?

Is the motor fitted high enough on the transom?

Pictures of the boat from the side showing the motor trimmed level ie mid position on the trailer would be handy too.

Cheers
Chimo

[Dignity]

Or is it because you have a new motor and you are looking back where as before you never gave it a thought, if that isn't the case then ignore this post and read all the other ones.

[aussiebasser]

Trimming the motor out won't help. You have to slow down gradually or you will wear a wave over the back. Trimming the motor out pushes the nose of the boat up and the stern down, making the wave effect worse. Trimming the motor in, tacks the motor in, makes the prop drive parallel with the bottom of the hull and stops the nose lift, bum drag effect.

[BrisFisher12]

Thanks for all the feedback guys, its great.


To answer a few questions:


Yes it does go faster on the plane, it's fantastic once up on the plane.


No trim tabs, No foil, It's a CoxCraft Rum Runner 2, I'm pretty sure the prop sits at the same level as the previous motor, we had to get the larger legged model of the motor. I don't have any pics at the moment, but can take some when I'm back home.


I'm definately paying more attention to the back of the boat now, I'm probably a little hyper sensitive. Have been doing lots of speed up / slow down running the motor in so I think that adds to it.


Just so I understand properly, when I'm ready to get up on the plane, I'll have the motor trimmed in (closest to the boat, so the prop is parrellel to the boat), then I'll slowly push forward on the throttle, at about 3500 rpm there is a wave forming just behind the motor, the bow is sitting up a fair bit at this point, then at about 3900-4000 it gets over the hump and is on the plane, the bow is now in a great position and the speed picks up. From what I've read I think that I can trim the motor out a little at this point, I haven't tried that yet.


Coming off the plane I drop the throttle to about 3500 rpm and it comes down, that wave just behind the motor forms, if I were to drop back to neutral it would hit the cowling i'm pretty sure. If I want to continue moving at 3500 rpm I can trim out more and reduce the wave as the bow of the boat is sitting up a fair bit at this point and trimming out brings the prop more parrellel to the boat.


If I had to stop suddenly do I just wear the wave hitting the stern? Am I concerned over nothing, is my method of operation normal?

[Chimo]

What length is the boat?

What HP and weight motor is the hull rated to carry?

What weight is the motor you have just fitted?

Who fitted the motor?

Did he water test it and try different props?

Have you been able to insure the boat with the new motor on it?

Ideally you should be able to trim the motor out / back from when the boat is on the plane. Realistically once your up and on the plane if you trim out the engine revs will pick up as you trim the motor up and back and so should your speed.

If this isn't the case the motor may be mounted too low.

To get an idea if the motor is at the correct height visualize the boat up and planing with someone else driving and you at the stern looking at the anti ventilation plate.
If your arm was long enough (and it isn't so don't try) you should be able to place your flat hand on the water surface between the bottom of the plate and the top of the spinning prop. If the plate is well under the water the motor probably needs to come up.

Cheers
Chimo

[Getout]

You will probably find that if you throttle off suddenly, the hull rides up on the following wave anyway with no harm done. Think about how much water slops around an outboard when in full reverse.